We investigated the dynamics and timing of left ventricular ejection in patients with hypertrophic cardiomyopathy (HCM), compared to patients with valvular aortic stenosis (AS), using radionuclide angiography. Patients with obstructive HCM had prolongation of left ventricular ejection time that was equivalent to patients with AS but significantly greater than patients with HCM without gradients. Despite prolonged ejection times, obstructive HCM patients had a period of brief, rapid ejection, with time to peak ejection rate values less than normal and equivalent to patients without gradients. Of note, 40% of patients with AS had supranormal ejection fractions. Hence, 1) supranormal EF does not exclude the possibility of significant outflow obstruction in HCM or AS, and 2) the outflow gradient in HCM represents true impedance to left ventricular emptying which occurs after an initial period of rapid ejection.